The invention relates to dialysis machines, and more particularly, to a negative pressure valve system and to alarm systems for use in such machines.
In dialysis, a patient's blood and dialysate flow through a dialyzer which includes a semipermeable membrane for separating the blood and the dialysate. Impurities and water from the blood cross the membrane and enter the dialysate for disposal. The terms dialysate, dialysis solution and dialyzing fluid as may be used hereinafter are intended to be synonomous.
In some dialyzers the dialysate is drawn through the dialyzer under a negative pressure (i.e., below atmospheric pressure). Such systems normally include a negative pressure pump positioned downstream of the dialyzer for drawing the dialysate through the dialyzer and a negative pressure valve positioned upstream of the dialyzer. The negative pressure in the dialyzer is controlled by adjustment of the negative pressure control valve. Although these systems are commonly referred to as negative pressure systems, there are certain conditions under which positive dialysate pressures may be generated. U.S. Pat. No. 3,878,095 discloses one such negative pressure system.
In some machines electromechanically operated needle valves have been used as the negative pressure control valve. Such valves have an operating characteristic such that as the valve moves from the open position toward the closed position, the change in pressure is relatively small and linear. However as the valve is about to close, the pressure becomes increasingly negative at a very rapid rate until the valve closes. In other words, as the valve closes, there are very great changes in pressure. This steep change in pressure makes it difficult to accurately control and maintain the negative pressure at highly negative levels (e.g., -400 to -500 mm Hg). This is particularly true in an electromechanical system wherein gear tolerances and changes in temperature also affect the control and positioning of the needle valve and thus the negative pressure.
Furthermore, the electromechanical system includes a constant speed DC motor to operate the valve. Therefore, since the valve characteristics are relatively linear, the time necessary to induce large changes in negative pressure may be relatively long. For example, the change from -200 mm Hg to 0 mm Hg may take on the order of two minutes.
It is therefore an object of this invention to provide a more accurately controllable and more responsive negative pressure valve system.
In dialysis the pressure differential across the semipermeable membrane (i.e., the difference in pressure between the blood and the dialysate) is important. This differential may be referred to as the transmembrane pressure. However, in the event that the dialysate pressure exceeds the blood pressure, impurities in the dialysate could undesirably pass through the membrane and into the blood.
It is desirable, therefore, that dialysis be prevented in the event that the dialysate pressure exceeds the blood pressure.
During dialysis, water is removed from the blood by a process known as ultrafiltration. The quantity of water removed is directly related to the transmembrane pressure. It is desirable to control the amount of water removed since removal of too much water during dialysis may result in undesirable side effects.
Therefore, it is desirable to maintain control over the difference between the dialysate pressure and blood pressure so as to control ultrafiltration.
Some prior art dialysis machines have included transmembrane pressure monitors, which merely measured and displayed the transmembrane pressure. In another machine, provisions were made for alarms in the event the transmembrane pressure exceeded a predetermined value. The alarms included a tolerance or alarm window of, for example, 50 mm Hg above or below the predetermined value. Therefore, in the event that the transmembrane pressure was zero, it is possible that with those tolerances dialysate pressure could increase beyond the blood pressure level, thereby permitting undesirable transfer from the dialysate to the blood.
It is therefore an object of this invention to provide an alarm system for preventing dialysis if the dialysate pressure exceeds the blood pressure.
In order to maintain a set or predetermined transmembrane pressure, both the dialysate pressure and the blood pressure must be monitored. In the event that the blood pressure signal is not received by the transmembrane pressure control system, it is possible that the actual transmembrane pressure could undesirably exceed the set or predetermined pressure without providing any indication or alarm as to that actual increase.
It is therefore another object of this invention to provide a system whereby the actual transmembrane pressure is maintained at a set or predetermined level, and in the event of signal failure from the venous pressure transducer, appropriate alarms and shut-off mechanisms are activated.
These and other objects of the invention will become apparent in the following description and appended claims.